If you mean the gracedia flower, you can't. The only way was to win it in an online event. The winners received the flower. Sorry
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If I'm correct, the Gracidea flower cannot be obtained in Pokemon Diamond. Sorry! (Note: The Gracidea can be obtained in Pokemon Platinum.)
It is impossible to find the Gracidea Flower in Pokemon Diamond and Pearl. It is available in Platinum.
surf there
you can't you can only get it on Pokemon platinum
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